1. Field of the Invention
This invention relates to a method of and apparatus for obtaining a radiation image in which a radiation image recorded on a radiation image conversion panel is obtained as a digital image signal.
2. Description of the Related Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, xcex1-rays, xcex2-rays, xcex3-rays, cathode rays or ultraviolet rays, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. As a system alternative to the conventional radiography using photographic film and a sensitized screen, a radiation image recording system using such a stimulable phosphor has been put into practice. In this system, a radiation image conversion panel (sometimes called xe2x80x9cstimulable phosphor sheetxe2x80x9d) having thereon a stimulable phosphor layer is used. That is, when the radiation image conversion panel is exposed to a radiation passing through an object or emitted from an object, the stimulable phosphor layer absorbs a part of energy of the radiation and stores a radiation image of the object as a latent image. When the radiation image conversion panel is exposed to stimulating light such as visible light or infrared rays, the stimulable phosphor layer emits stimulated emission in proportion to the radiation energy stored thereon. The stimulated emission is photoelectrically converted into an electric image signal and a radiation image of the objected is reproduced as a visible image on the basis of the electric image signal. After the radiation image is read out the radiation image conversion panel is exposed to erasing light and is caused to release residual radiation energy so that the radiation image conversion panel can be used again.
It is preferred that the radiation image conversion panel be highly sensitive and can reproduce a high quality radiation image. Especially, in the diagnostic radiation image recording and reproducing system, which is a typical application of the radiation image recording system using the radiation image conversion panel, a high quality image is desired.
However, the stimulating light projected onto the stimulable phosphor layer of the radiation image conversion panel when the radiation image stored thereon is to be read diffuses in the stimulable phosphor layer and deteriorates resolution of the radiation image. That is, the latent radiation image stored on the stimulable phosphor layer of the radiation image conversion panel is read out by causing a beam of the stimulating light focused on the surface of the radiation image conversion panel to scan the radiation image conversion panel in both a main scanning direction and a sub-scanning direction and detecting the stimulated emission emitted in sequence from the parts of the radiation image conversion panel as the stimulating light beam scans the radiation image conversion panel. When the stimulating light two-dimensionally diffuses inside the phosphor layer, the stimulable phosphor outside the scanning spot is also stimulated to emit stimulated emission, which deteriorates the resolution of the radiation image.
In order to avoid such a phenomenon, it has been proposed to provide a partition wall impermeable to the stimulating light in the stimulable phosphor layer to linearly or two-dimensionally part the stimulable phosphor layer into a plurality of regularly arranged fine cells. In such an arrangement, stimulating light impinging upon one cell is prevented from diffusing to adjacent cells by the partition wall which is impermeable to the stimulating light, whereby a radiation image having a high resolution can be read out.
The partition wall impermeable to the stimulating light need not be perfectly impermeable to the stimulating light so long as it is less permeable to the stimulating light as compared with the stimulable phosphor in the cell.
However, the system provided with the partition wall impermeable to the stimulating light is disadvantageous in that each time the stimulating light beam passes the partition wall, the intensity of the stimulated emission becomes weak at regular intervals, and the analog image signal obtained by photoelectrically converting the stimulated emission into an electric signal comes to include periodic signals (e.g., signals which will generate moirxc3xa9 fringes) which represents that the stimulating light beam passes the partition wall. When the analog image signal is digitized into a digital image signal, aliasing noise can be generated to distort the image depending on the relation between the periodic signals generated when the stimulating light beam passes the partition wall and the sampling periods at which the analog image signal is sampled.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and an apparatus for obtaining a digital radiation image which is free from aliasing noise which is generated when an analog image signal read out from a radiation image conversion panel provided with a partition wall is digitized and which can reproduce a high quality radiation image.
In accordance with a first aspect of the present invention, there is provided a method of obtaining a radiation image in which a radiation image conversion panel having a stimulable phosphor layer which is parted into a plurality of cells regularly arranged at least in a main scanning direction by a partition wall and has been exposed to a radiation bearing thereon a radiation image of an object is scanned by a stimulating light beam in the main scanning direction and a sub-scanning direction, stimulated emission emitted from the radiation image conversion panel upon stimulation by the stimulating light beam is photoelectrically detected to make up an analog image signal and a digital image signal representing a radiation image of the object is obtained by digitizing the analog image signal, wherein the improvement comprises that
the sampling intervals Ts (in seconds) for digitizing the analog image signal are set to satisfy formula
Tsxe2x89xa6Na/2v,
wherein Na represents the pitch (m) of the cells in the main scanning direction and v represents the scanning speed (m/sec).
It is preferred that the sampling intervals Ts be set to satisfy formula
Na/40vxe2x89xa6Tsxe2x89xa6Na/2v.
It is further preferred that the analog image signal be obtained through a low-pass filter whose cut-off frequency fc (Hz) satisfies formula
fcxe2x89xa60.5/Ts.
In accordance with a second aspect of the present invention, there is provided a method of obtaining a radiation image in which a radiation image conversion panel having a stimulable phosphor layer which is parted into a plurality of cells regularly arranged at least in a sub-scanning direction by a partition wall and has been exposed to a radiation bearing thereon a radiation image of an object is scanned by a stimulating light beam in a main scanning direction and the sub-scanning direction, stimulated emission emitted from the radiation image conversion panel upon stimulation by the stimulating light beam is photoelectrically detected to make up an analog image signal and a digital image signal representing a radiation image of the object is obtained by digitizing the analog image signal, wherein the improvement comprises that
the scanning intervals L (m) in the sub-scanning direction are set to satisfy formula
Lxe2x89xa6Nb/2,
wherein Nb represents the pitch (m) of the cells in the sub-scanning direction.
It is preferred that the scanning intervals L be set to satisfy formula
Nb/40xe2x89xa6Lxe2x89xa6Nb/2.
When the radiation image conversion panel has a stimulable phosphor layer which is parted into a plurality of cells regularly arranged in both a main scanning direction and a sub-scanning direction by a partition wall, it is preferred that
the sampling intervals Ts (in seconds) for digitizing the analog image signal be set to satisfy formula
Tsxe2x89xa6Na/2v,
and at the same time,
the scanning intervals L (m) in the sub-scanning direction be set to satisfy formula
Lxe2x89xa6Nb/2,
wherein Na represents the pitch of the cells in the main scanning direction, v represents the scanning speed (m/sec) and
Nb represents the pitch (m) of the cells in the sub-scanning direction.
At this time, it is further preferred that the sampling intervals Ts be set to satisfy formula
Na/40vxe2x89xa6Tsxe2x89xa6Na/2v
and the scanning intervals L be set to satisfy formula
Nb/40xe2x89xa6Lxe2x89xa6Nb/2.
In accordance with a third aspect of the present invention, there is provided an apparatus for obtaining a radiation image comprising a radiation image conversion panel having a stimulable phosphor layer which is parted into a plurality of cells regularly arranged at least in a main scanning direction by a partition wall, a scanning means which causes a stimulating light beam to scan the radiation image conversion panel in the main scanning direction and a sub-scanning direction, a detecting means which photoelectrically detects stimulated emission emitted from the radiation image conversion panel upon stimulation by the stimulating light beam to make up an analog image signal and a digital image signal obtaining means which digitizes the analog image signal into a digital image signal representing a radiation image of the object, wherein the improvement comprises that
the digital image signal obtaining means samples the analog image signal at sampling intervals Ts (in seconds) which are set to satisfy formula
Tsxe2x89xa6Na/2v,
wherein Na represents the pitch of the cells in the main scanning direction and v represents the scanning speed (m/sec)
It is preferred that the sampling intervals Ts be set to satisfy formula
Na/40vxe2x89xa6Tsxe2x89xa6Na/2v.
It is further preferred that said detecting means obtains the analog image signal through a low-pass filter whose cut-off frequency fc (Hz) satisfies formula
fcxe2x89xa60.5/Ts.
It is further preferred that the pitch of the cells in the main scanning direction be in the range of not smaller than 20 xcexcm and not larger than 300 xcexcm.
In accordance with a fourth aspect of the present invention, there is provided an apparatus for obtaining a radiation image comprising a radiation image conversion panel having a stimulable phosphor layer which is parted into a plurality of cells regularly arranged at least in a sub-scanning direction by a partition wall, a scanning means which causes a stimulating light beam to scan the radiation image conversion panel in a main scanning direction and the sub-scanning direction, a detecting means which photoelectrically detects stimulated emission emitted from the radiation image conversion panel upon stimulation by the stimulating light beam to make up an analog image signal and a digital image signal obtaining means which digitizes the analog image signal into a digital image signal representing a radiation image of the object, wherein the improvement comprises that
the scanning means causes the stimulating light beam to scan the radiation image conversion panel in the sub-scanning direction at scanning intervals L (m) which are set to satisfy formula
Lxe2x89xa6Nb/2,
wherein Nb represents the pitch (m) of the cells in the sub-scanning direction.
It is preferred that the scanning intervals L be set to satisfy formula
Nb/40xe2x89xa6Lxe2x89xa6Nb/2.
It is further preferred that the pitch of the cells in the sub-scanning direction be in the range of not smaller than 20 xcexcm and not larger than 300 xcexcm.
When the radiation image conversion panel has a stimulable phosphor layer which is parted into a plurality of cells regularly arranged in both a main scanning direction and a sub-scanning direction by a partition wall, it is preferred that
the sampling intervals Ts (in seconds) for digitizing the analog image signal be set to satisfy formula
Tsxe2x89xa6Na/2v,
and at the same time,
the scanning intervals L (m) in the sub-scanning direction be set to satisfy formula
Lxe2x89xa6Nb/2,
wherein Na represents the pitch (m) of the cells in the main scanning direction, v represents the scanning speed (m/sec) and
Nb represents the pitch (m) of the cells in the sub-scanning direction.
At this time, it is further preferred that the sampling intervals Ts be set to satisfy formula
Na/40vxe2x89xa6Tsxe2x89xa6Na/2v
and the scanning intervals L be set to satisfy formula
Nb/40xe2x89xa6Lxe2x89xa6Nb/2.
It is further preferred that the pitch of the cells be in the range of not smaller than 20 xcexcm and not larger than 300 xcexcm in both the main scanning direction and the sub-scanning direction.
In accordance with the first aspect of the present invention, since the sampling intervals Ts (in seconds) for digitizing the analog image signal are set to satisfy formula
Tsxe2x89xa6Na/2v,
sampling is effected two or more times in a pitch of the cells in the main scanning direction and accordingly, regular signal components of the analog image signal representing the partition wall are digitized under a condition which satisfies the so-called sampling theorem, whereby the regular signal components representing the partition wall are prevented from being included in the digital image signal as aliasing noise.
Further, when the sampling intervals Ts are set to satisfy formula
Na/40vxe2x89xa6Tsxe2x89xa6Na/2v,
unnecessary signal components which are excessively high in frequency are not included in the digital image signal, and accordingly, the signal processing time can be shortened and load on the system can be lightened.
When the analog image signal is obtained through a low-pass filter whose cut-off frequency fc (Hz) satisfies formula
fcxe2x89xa60.5/Ts,
generation of aliasing noise other than those due to the regular signal components representing the partition wall is prevented.
When the pitch of the cells in the main scanning direction is in the range of not smaller than 20 xcexcm and not larger than 300 xcexcm, the cells can be easily formed while holding desired resolution.
In accordance with the second aspect of the present invention, since the scanning intervals L (m) in the sub-scanning direction are set to satisfy formula
Lxe2x89xa6Nb/2,
sampling is effected two or more times in a pitch of the cells in the sub-scanning direction and accordingly, regular signal components of the analog image signal representing the partition wall are digitized under a condition which satisfies the so-called sampling theorem, whereby the regular signal components representing the partition wall are prevented from being included in the digital image signal as aliasing noise.
Further, when the scanning intervals L are set to satisfy formula
Nb/40xe2x89xa6Lxe2x89xa6Nb/2,
unnecessary signal components which are excessively high in frequency are not included in the digital image signal, and accordingly, the signal processing time can be shortened and load on the system can be lightened.
When the radiation image conversion panel has a stimulable phosphor layer which is parted into a plurality of cells regularly arranged in both a main scanning direction and a sub-scanning direction by a partition wall, the sampling intervals Ts (in seconds) for digitizing the analog image signal are set to satisfy formula
Tsxe2x89xa6Na/2v,
and at the same time, the scanning intervals L (m) in the sub-scanning direction are set to satisfy formula
Lxe2x89xa6Nb/2,
sampling is effected two or more times both in a pitch of the cells in the main scanning direction and in a pitch of the cells in the sub-scanning direction and accordingly, regular signal components of the analog image signal representing the partition wall are digitized under a condition which satisfies the so-called sampling theorem, whereby the regular signal components representing the partition wall are prevented from being included in the digital image signal as aliasing noise.